Homogeneous and biphasic cellulose acetate/room temperature ionic liquid membranes for gas separations: Solvent and phase-inversion casting vs. supported ionic liquid membranes

Abstract

Standard casting methods, using polymer/RTIL solutions, can produce membranes with improved stability. We evaluated the performance of RTIL-membranes cast using either solvent casting (homogeneous polymer/RTIL film) or phase-inversion (biphasic polymer/RTIL films). The study used a model casting polymer, cellulose acetate (CA), and the RTIL 1-ethyl-3-methylimidazolium thiocyanate, ([emim][SCN]). The gas separation performances of the resulting homogeneous and biphasic CA/[emim][SCN] membranes were compared with a conventional supported ionic liquid membrane (SILM) of [emim][SCN]. The evaluations of CO2 removal from CH4 supported our hypotheses that the biphasic phase-inversion cast membranes have higher selectivities and membrane stability compared to the homogeneous film membranes. While the homogeneous membranes had the highest gas permeances, the biphasic membranes had three-fold higher selectivities. The highest CO2/CH4 mixed-gas selectivity reported was 89 ± 12 for the biphasic membrane. It appears that selectivity in cast membranes is a function of free RTIL content. The order of membrane stability vs. cross-membrane pressure was biphasic ≈ SILM > homogeneous, with the biphasic breakthrough pressure being two-fold higher than the homogeneous membrane's. Therefore, the phase-inversion casting method (biphasic membranes) is a viable alternative to SILM membranes for fabricating RTIL-membranes. Infrared spectra and atomic force micrographs characterized the homogeneous and biphasic membrane morphologies.